1. Field of the Invention
The field relates generally to separation of a wafer into a plurality of die, and in particular relates to separation of a wafer into a plurality of die where the die have thereon surface critical devices, such as the structures for microelectromechanical systems.
2. Description of the Related Art
Microelectromechanical systems (MEMS) in some instances are fabricated using relatively standard wafer fabrication techniques. In such processing, a plurality of die are fabricated from each wafer, and the wafer must, at or near the end of the fabrication process, be separated into a plurality of the die comprising the MEMS device.
However, the MEMS structures created on the die can be quite fragile in comparison with other types of circuits and devices fabricated using similar techniques. In particular, in a number of MEMS devices, part of the fabrication process is to create a plurality of thin metallized ribbons (or functionally similar shapes) above the substrate of the wafer. The ribbons are initially fabricated by depositing metal (usually, though not necessarily aluminum) on a sacrificial layer of silicon that is deposited on a substrate. After the ribbons are defined by the metal deposition steps, the sacrificial layer of silicon is removed by an etching process, leaving the ribbons free xe2x80x9creleasedxe2x80x9d so that they can move toward the substrate by the application of a voltage potential between the ribbon and the substrate.
Following the completion of processing, the wafer, which typically contains as many as several hundred individual devices, must be separated into the individual die. In conventional semiconductor processing, this is accomplished by a dicing saw, where the saw is aligned with the wafer from the top and the wafer is then cut. In conventional dicing, a digital camera can image location indicators on the front side of the wafer. These location indicators can be etched in the same layers used to create the devices. The location indicators are typically used to line up the saw blade by adjusting the position and rotation of the wafer on the dicing chuck of the saw. A water bath is typically applied to keep the saw and wafer cool, such that water and silica dust are allowed to coat the surface of the device.
However, such processing techniques would yield only disaster for certain types of MEMS devices, because water and silica dust are major contaminants which can prohibit the proper deflection of the ribbons as well as other problems. Certain MEMS devices present unusual dicing issues because, once the ribbons have been released, they become particularly fragile and susceptible to contamination. In addition, no photoresist or other protective layer can be placed over the MEMS structures (e.g., the ribbons) during the dicing process, unlike more conventional semiconductor devices.
A vacuum chuck is typically used to hold the wafer during dicing. A conventional dicing chuck uses concentric vacuum rings to secure the wafer to the chuck. In such an arrangement, water can wick underneath the wafer, even across the entire surface, which can introduce capillary forces that draw water into the space between the ribbons and the substrate. Removing the water from the devices can be difficult even after the vacuum has been turned off.
As a result, new devices, methods and techniques must be developed to ensure that the MEMS devices located on the surface of the wafer are not damaged while separating the wafer into die.
The present invention teaches devices and techniques for separating a wafer on which MEMS devices have been fabricated into the die containing those devices. In particular, the present invention can include the following general steps. First, an alignment jig is developed so that the backside of the wafer can be aligned with the devices located on the upper or active front side of the wafer. Then, the active area of the wafer is placed against the chuck of the dicing saw; in some embodiments a series of pillars or stand-offs may be provided to allow the wafer to be mounted to the chuck at a height greater than the height of the MEMS structures. The alignment jig permits the wafer to be aligned in a manner similar to conventional front side alignment, but entirely from the back. The back of the wafer is then, in at least some embodiments, scribed to establish alignment marks.
Next, the wafer is saw is used from the back side to cut or dice part way through the wafer along the rows and columns which define the various die. By cutting only part way through, the presence of silica dust and water is minimized on the active surface of the wafer. A special vacuum chuck arrangement may be provided to seal the front surface of the wafer against water and silica dust, including the use of a sealing material which may be provided at the outer edge of the wafer to further prevent unwanted contamination of the active area of the front of the wafer. The backside of the wafer is then cleaned after the partial saw cuts are made.
Next, a layer of stretchable material, for example a stretchable tape, is applied to the backside of the wafer so that each of the die adheres to the tape. The tape is then stretched, causing the wafer to separate along the various partial saw cuts made from the back side of the wafer, which define the individual die. The die are then removed from the tape, which may be accomplished in at least some embodiments by exposing the tape to ultraviolet light.
These and other details may be better appreciated from the following Detailed Description of the Invention, taken in light of the accompanying Figures.